Electroplating of copper from cyanide electrolytes



Patented Apr. 28, 1953 ELECTROPLATING OF COPPER FROM CYANIDE ELECTROLYTES George W. J ernstedt and James 1). Patrick, Pittsburgh, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application February 18, 1949, Serial No. 77,265

3 Claims.

This invention relates to the electrodeposition of copper by means of periodic reverse current from cyanide electroplating electrolytes containing certain joint addition agents.

-In Patent 2,451,341, there is disclosed the process of electroplating copper from an electrolyte containing copper in solution by employing a periodic reverse electric current. Specifically, a member to be plated with copper is immersed in the electroplating electrolyte and subjected to a flow of electrical current such that for a period of time of not over 40 seconds a plating electric current deposits copper on the member, and then for a shorter period of time a deplating electric current of such a magnitude is applied to the member that it delivers at least of the coulombs of electric current delivered in the preceding plating period whereby a part of the plated copper is removed thereby leaving an increment composed of sound, smooth and relatively-uniform copper plate. Repetition of the alternate plating and deplating builds u the copper until a desired thickness of copper has been applied to the member. The copper plate so produced on the member is sounder, harder, smoother and more homogeneous than copper applied by previously-known processes. Numerous other unexpected advantages have been realized over any previously-known method of electroplating.

Specifically, periodicreverse current enables the electroplating of copper to be so carried out that one or more of the following advantages over continuous direct-current plating are obtained.

(7) Improved electrodeposited metal distribution on base members.

The periodic reverse current process may be so carried out that any one feature of the above seven can be emphasized. While this gives the electroplater a control of the plating process that was not available when employing only continuous direct current, or other conventional plating current cycles disclosed in the prior art, there are instances where it is desirable to emphasize two or more of the above features at the same time. In particular, it is desirable to plate at the highest possible rate of copper deposition while producing a smooth, bright deposit. Obviously, there is a limit to the current density at which plating may be carried out while maintaining the resulting electrodeposit of metal smooth and bright, regardless of the mode of plating.

These are suggestions in the prior art that copper may be electrodeposited by direct current from a copper cyanide electrolyte while dissolved zinc is present in the electrolyte; see the article by Walter R. Meyer and Arthur Phillips on pages 400 and 401 of the Transactions of the Electrochemical Sooiety, vol. LXXIII (1938). However, this article indicates that the sole benefit from zinc additions in copper electrolytes is a slight increase in brightness for very thin electrodeposits not thicker than one-half mil (0.0005) inch), while any thicker deposits become inferior having a distinct columnar structure with rougher surfaces than that of electroplated deposits of equal thickness obtained from the same copper electrolyte with no zinc present. The use of thiocyanate addition agents alone to copper cyanide electroplating electrolytes has been suggested previously for direct-current electroplat ing. Thiocyanate has been employed in directcurrent electroplating with moderate benefits; however, other addition agents give superior results, and thiocyanates are not used to any great extent as an addition agent for copper cyanide electrolytes.

In accordance with this invention, it has been discovered that the electrodeposition of copper from aqueous cyanide electrolytes containing copper by periodic reverse current may be greatly improved by having present simultaneously in the electrolyte zinc and thiocyanate (CNS-L When operated with periodic reverse current, the presence of both zinc and the thiocyanate in particular proportions in copper cyanide electrolytes produces results markedly superior than with either alone.

The object of this invention is to provide for the electrodeposition of copper from aqueous copper cyanide electrolytes containing zinc and thiocyanate jointly in predetermined amounts, by means of a periodic reverse electric current.

A further object of this invention is to provide a process for the electrodeposition of smoother and brighter deposits of copper from cyanide electrolytes by maintaining predetermined amounts of both zinc and thiocya gin the electrolyte and passing a periodic reverse current through the electrolyte.

A still further object of the invention is to provide aqueous copper cyanide electroplating electrolytes containing, predetermined amounts of both zinc and thiocyanate therein.

Another object of the invention is to provide an addition agent containing both zinc and thiocyanate capable of being readily dissolved in aqueous solutions to promote plating from copper cyanide electrolytes.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

It has been discovered that electrodeposition of copper by means of periodic reverse current from a copper cyanide electroplating electrolyte containing both zinc and thiocyanate in predetermined amounts enables markedly improved copper electrodeposits to be obtained as compared to the same electrolyte without the addition agents or with the electrolyte containing either one of the addition agents zinc or thiocyanate alone. While ordinarily the combination of two or more addition agents in an electrolyte does not improve electrodeposition and, in fact, in some cases impairs the electrodeposition and only in rare cases improves electrodeposition, as compared to the results secured with either agent used alone, unexpected results are obtained from the combination of zinc and thiocyanate addition agents with periodic reverse current.

Specifically, outstanding copper electrodeposits have been obtained from cyanide copper electrolytes containing jointly from 0.004 to 0.5? ounce per gallon of dissolved zinc and from 0.1 to

ounces per gallon of thiocyanate (CNS-) in the electrolyte. The optimum proportions have been found to be from 0.05 to 0.2 ounce per gallon of zinc and from 0.5 to 2.0 ounces per gallon of thiocyanate. Tests have shown that the presence of zinc in amounts greater than 0.5? ounce per gallon is detrimental to the plating, while amounts of zinc less than 0.004 ounce per gallon produces no benefit. Thiocyanate present in an amount less than 0.1 ounce per gallon gives no benefits, while above 10 ounces per gallon its benefits decrease markedly. When a periodic reverse current is employed to electrodeposit copperfrom a copper cyanide electrolyte having both of these addition agents presentin the effective range of proportions, smooth, hard, bright and uniform depositsof copper .have been obtained over a wide range of current densities. In all cases, the quality of the copper has been superior to that producedby direct current from any known electrolyte. Many electrodeposits of copper have been secured by .the practice of the present invention that .have unique characteristies as will be set forth in detail hereinafter.

.The electrolytes from which copper is'to be electroplated may be potassium cyanide electrolytes, sodium cyanide electrolytes, mixed potassium and sodium cyanide electrolytes, and Rochelle copper cyanide electrolytes; and when the zinc and thiocyanate are introduced therein in the proportions herein set forth, the electrodeposition ofcopper by periodic reverse current is markedly benefited.

A copper cyanide electroplating electrolyte may be prepared with from 2 to ounces per gallon of dissolved copper, an alkali metal cyanide in an amount to complex the copper in solution, thatis, to produce the more soluble alkali metal-copper cyanide double salt, and in addition to provide .for 0.5 to 5 ounces pergallonof free alkali metal cyanide, from 2 to 8 ounces per gallon of alkali metal hydroxide, from 1 to 20 ounces per gallon of alkali metal carbonate, from 0.004 to 0.57 ounce per gallon of zinc and from 0.1 .to 10 ounces per ,gallon of thiocyanate (CNS). The pH ofthe cyanide solutions is usually maintained at 12 to 13 and slightly higher. The electrolyte may contain other components commonly present in cyanide copper electroplating electrolytes such, for example, as '3 to 8 ounces per gallon of Rochelle salts (potassium sodium tartrate).

The zinc may .beintroduced into the electrolyte as one or more of a number of zinc salts such, for example, as zinc oxide, zinc cyanide, or zinc thiocyanate; or it may be added by introducing a'zinc anode into the electrolyte and passing a deplating current to dissolve the zinc in the solution. The thiocyanate may be added as an alkali metal thiocyanate such, for example, as sodium or potassium thiocyanate, or as a copper thiocyanate, or as a zinc thiocyanate salt. We have found thatthe following prepared compositions are most satisfactory for introducing both the zinc and thiocyanate into the electrolyte: A composition of from 0.5 to 2 parts by weight of zinc oxide, from 1 .to 10 parts by weight of copper thiocyanate, andsufficient alkalimetal cyanide to render the zinc oxide and copperthiocyanate readily soluble in water,,the alkali metal cyanide not exceeding 20 parts by weight. The following specific composition has been used with success: 1 part zinc oxide, 2 parts copper.thiocyanate and 5 parts sodium cyanide. When introducedinto'an electrolyte in the proportions of 1 ounce pergallon, this specific composition provides close to 0.1 ounce .per gallon of zinc and 0.18 ounce pergallon of thiocyanate. There is sufficient sodium cyanide present to render the zinc oxide and copper thiocyanate readily soluble in the aqueous electrolyte.

Another composition that has been found satisfactory is prepared by combining 1 part of either zinc oxide or zinc cyanide, or va mixture of both, with from v1 to 25 parts by weight of an alkali metal thiocyanate such .as. sodium thiocyanate. or potassium thiocyanate.

One important operating .advantage derived by the use of combined zincand thiocyanate additions in copper cyanide electrolytes is that the resulting electrolyte .canbe treated with activated. carbon and filtered with very 'little .loss of either zinc or thiocyanate. Metal thiocyanates are considered to be inorganic compounds. The commonly used organic addition agents, which are usually quite expensive, are removed rather completely from copper cyanide electrolytes when subjected to the same treatment. Therefore, the expense of replenishment of removed addition agent is avoided. Because of this fact, the electrolytes of the present invention can be economically treated with activated carbon, at any time to remove impurities and thereby are maintainable in most efiicient condition.

As disclosed in the patent to Jernstedt, No. 2,451,341, .a member to .be plated by periodic reverse current in the electroplating,electrolytes of this invention has applied to it a plating electric current, to render themember a cathode, for plating copper from theelectrolyte thereonfor a period of time of not over 40 seconds. With the electrolytes ofthe present invention, the plating period may be as short as /mof .a second. After an increment of copper has been plated on the member, then .a depleting electric current is :ap-

ni ce-t the mem er.- wi'i nd tr a teseber'.;-;;Repetition1'of the plating and deplating,

electric current cycle, builds up a progressively smoother and more uniformcopperelectrodeposit on the member. It has been found that for some purposes the deplating current may be as much as 75% of the plating current or higher with exceptionally gooddeposits being secured. The ratio of coulombs of plating current to deplating current may be varied in accordance with" the required quality, such as. brightness, smoothness or uniformity, of thedeposit. The deplating current density will ordinarily be the same as the plating current density since it is-most convenient in the majority of plating applications-to reverse a relatively-constant, continuous direct current by means of suitable switches and timers. However, advantages have been found to accrue if the deplating current density is greater than the plating current density, The deplating current density may be double, triple oreven higher than the plating current density with advantages arising. However, the deplating current density cannot be reduced to below 75% of the plating current density without loss of benefits of the periodic reverse current benefits at the preferred conditions. The following examples are illustrative of thepractice of the invention: U I v Example I An aqueous copper cyanide-electrolyte of th following composition was preparedzr Copper (as a metal) 6 ounces per gallon. Potassium hydroxide 6 ounces per gallon.

Free potassium cyanide 1 ounce'per gallon.

Zinc cyanide Suflicient to provide 0.1 ounce per gallon of zinc.

Potassium thiocyanate Sufiic ent to provide 0.5 ounce per gallon of thiocyanate (CNS-L The temperature of the'electrolyte was 180. F. Copper was electroplated from this electrolyte using a periodic. reverse current of seconds plating and 1 second deplating. In the, range of 13 to 105 amperes per square foot, practically mirror-bright copper was produced from the electrolyte; from 105 to 135 amperes per square foot, mirror bright copper plate that was extremely smooth and uniform resulted; from 135 to 160 amperes per square foot, the copper elec-' trodeposit again was nearly mirror bright.

' The zinc content of the electrolyte of Example I was increased to 0.2 ounce per gallon'of zinc,'

Example II t i-lln'aqueous copper cyanide electrolyte was prepared with the following composition:

Ounces per gallon Copper (as a'metal) 8 Potassium hydroxide 5 Free potassium cyanide -1- thiselectrolyte there was addedl ounce per gallon of a com sition. qmnr sies part y w e-htpf ZiIl IEOXid nZ. parts y: we ht .cfmnnper: thiocyanate, and 5 parts by weight of; sodiunr cyanide. A periodic reversej current having a cycle oi plating for 5 seconds and, deplating for 1 second'was employed forplating copper from this electrolyte on members. It produced mirrorbright'copper at current densities of up to amperes per square foot and very bright copper, plate up to and beyond amperesper square; foot. .v

The composition of Example II was employed for-plating heavy deposits of copper-using a pe-' riodicreverse-current of 15 seconds plating and 7: seconds deplating. In this case, the'work con-.v

sisted of a record matrix that was rotated in the electrolyte at 60 R. P. M.-with moderate agita-.v tion of the solution. The current density in both theplating and deplating cycles was 80 amperes per square foot. In 8 hours a 0.032 inch thick de posit of copper that was mirror bright and smooth was produced Heretofore, the best plat-' ing practice required from 18 to 24 hours to plate a deposit of copper on the record matrix, and theresulting copper deposit was quite nodular and rough so that it required machining to remove: beads 'andinodules from the edges and back sideof the plate. The plated deposit produced in ac-" cordance with the present invention required no; machining whatever. However, the main difierence's-between the best previous copper deposits and that of Example II. were the'x'outstanding: hardness and the superior physical characteristics of the copper deposit produced by the present invention. The copper plates produced in accordance with this Example were of a hard ness of over 10.0 on the Rockwell F scale, whereas previously a hardness of 70 was regarded as normali- Th copper plate produced in accordance with the present invention was elastic and could be bent considerably without permanent diston tio'n; whereas the previous direct-current de posits would tak'e'a permanent set if bent to the same extent.

'- When the. electrolyte of Example II was em-' ployed' for direct-current plating, the deposits.

- were of a light matte texture at current densi- A the zinc and .thiocyanate, the same electrolyte produced deposits with direct current having a medium fogged appearance when plated at current densities of 12 amperes per square foot, and dark matte from 12 to 30 amperes per square 7 foot, and light matte above 30 amperes per square foot.

, y Example III 7 "An electrolyte was prepared as in Example 11-- with an additional 0.1 ounce per gallon of zinc added as zinc oxide and 0.175 ounce per gallon of thiocyanate added as potassium thiocyanate. When plated with periodic reverse current having a cycle of 5 seconds plating and 1 second de-K plating, mirror-bright deposits were obtained at current densities of up to amperes per square foot. i

The electrolyte of Example III had added to it an additional 0.25 ounce per gallon of zinc thiocyanate. Bright deposits were obtained with periodic reverse current having cycles of 5 seconds plating and 1 second deplating at current densities of up to 150 amperes per square foot.

When. a further.0.5 ounce of zinc thiocyanate ea -coo 7 bright deposits were obtained up to i50xamperes per square foot.

Example IV An aqueous cyanide electrolyte was prepared with:

Ounces per gallon Copper 6 r e cy nide 0.75 Potassium hydroxide Potassium carbonate 5 To this electrolyte was added 1 ounce pe gallon of a composition composed of ,1 part zinc oxide, 2 parts of copper thiocyanate and 5 :par ts.0f'O'- dium cyanide, thereby'providing 0.1 ounce per gallon of zinc and 0.18 ounce of thiocyanate. A cylinder was plated in this electrolyte by immersing one third of the periphery in the elecetrolyte, .the cylinder being rotated at a speed of 225 revolutions per minute while being plated. The cylinder had applied to it a periodic reverse current'of 1-5 seconds plating and 3 seconds deplating at a current density of 300 amperes per square root of immersed area during both parts of the cycle. 'Intwo hours the entire surface .01 thecylinder was coated with 0.006 inch thick of copper. The plated copper was mirror bright and smooth. The surface roughness of the copper was approximately 2 micro-inches whereas the original cylinder surface had a roughness of more'than :8 micro-inches (rms) Example V An aqueous cyanide electrolyte of the following composition was prepared:

Ounces per ,gallon Copper 12.5 NaOH 7.5 Free NaCN 1:75 NasCOs 3.5 Zinc .0408

Thiocyanate (CNS-) 0,1 5

This electrolyte wasemployed for copper plating fabricated steel members .and various zinc die castings used in commercial automobiles using periodic reverse currents having the following time-periods:

Plating, Deplating, seconds seconds (a) *5 (o) 1-5 3 (c) 5 1 Plating, Depicting,

sec. sec.

flan 39m 1 ,0. 10 2 .20 l0 The electrolyte must be replenished with the addition agent particularly to" provide for mainvtaining the quantity of zincfa small part of dition agents within proper limits providing the zinc and .thiocyanate are added amounts at regular intervals.

It will be appreciated that the cyanide elecin suitable trolytes in the above examples are exemplary and not exhaustive. In general, it may be stated that any cyanide electrolyte from which copper may be plated by continuous direct current may be treated to add thereto both zinc and thiocyahate and used with periodic reverse current in accordance with this invention, with substantial improved plating obtained.

The electrolytes may be agitated or stirred with advantage while plating with periodic reverse-current. Filtering of the electrolytepreferably continuously, is recommended to secure electrodeposits of the optimum quality.

The preparation of the base members to be plated with periodic reverse cur-rent should be such as to produce a chemically clean surface as is conventional in order to secure the desired adherence of the metal to the base member. The preparation of the base member, therefore, may include brushing, grinding, sandblasting, shot blasting, degreasing, electrolytic cleaning and the like.

Various types and kinds of base members may be plated. Metallic bodies on which metal may be plated from conventional electrolytes may be electroplated by periodic reverse current. Carbon and graphite forms may be so electroplated. Electrotypes and electroforming base members consisting of wax or resinous patterns plated or coated with powdered graphite or precipitated silver may be electroplated as disclosed herein v and the electrodeposi-ted metal stripped therefrom. It will be found that electrotypes, molds and other electroformed members produced in accordance with the present invention will give a superior reproduction of the surface.

The copper electrodeposits produced by the use of periodic reverse current in accordance with the present invention may be plated subsequently with one or more other metals applied with similar periodic reverse. current or by any other desirable process, such as by continuous direct current. In many cases, it has been found that the outstanding smoothnessof the copper .electrodeposits of the present invention enables the plating ofsubsequent metals even .by continuous direct current or other conventional process .to great advantage since the smoothness of' the surface beneficially affects the subsequent plating. Thus, ,we have been able to plate an. initial layer of copper with periodic reverse current and thereafter plated, nickel and chromium by direct current, and the nickel and chromium were so bright that no bufiing was required. The periodic reverse current produces metal deposits that are so homogeneous and free from pores and other flaws that they possess an improved corrosion resistance for a given thickness ,over the a meta appl ed y c ntinuous direct current.

It is intended that all the matter contained in the above description shall be deemed to be illustrative and not limiting. Y

We claim as our invention:

1. In the process of electroplating on a member electrodeposits of copper from an aqueous copper cyanide electroplating electrolyte, the steps comprising applying to the member the aqueous cyanide electrolyte having dissolved therein from 2 to 15 ounces per gallon of copper, between 0.004 and 0.57 ounce per gallon of zinc and thiocyanate (CNS'-) in an amount of between 0.1 and 10 ounces per gallon, causin a plating electric current to how through the mem ber while in contact with the electrolyte for a period of time of not over 10 seconds to electroplate copper on the member, then causing a deplating electric current of a sufficient magnitude to flow through the member for a period of time to deliver at least 10% of the coulombs delivered during the preceding plating period to deplate a part or" the copper plated on the member, and continuing the alternate plating and deplating until a desired thickness of copper has been deposited on the member, the zinc and the thiocyana-te present in the electrolyte in the given proportions resulting in the plating of a brighter and smoother electrodeposit of the copper.

2. In the process of electroplating on a member electrodeposits of copper from an aqueous copper cyanide electroplating electrolyte, the steps comprising applying to the member the aqueous cyanide electrolyte having dissolved therein from 2 to 15 ounces per gallon of copper, between 0.05 and 0.2 ounce per gallon of zinc and thiocyanate (CNS*) in an amount of between 0.5 and 2.0 ounces per gallon, causing a plating electric current to now through the member While in contact with the electrolyte for a period of time of not over 40 seconds to electroplate copper on the member, then causing a deplating electric current of a sufiicient magnitude to flow through the member for a period of time to def liver at least 10% of the coulombs delivered during the preceding plating period to deplate a part metal cyanide, from 2 to 8 ounces per gallon of alkali metal hydroxide, from 1 to 20 ounces per gallon of alkali metal carbonate, from 0.004 to 0.5? ounce per gallon of zinc and from 0.1 to 10 ounces per gallon of thiocyanate, the zinc and thiocyanate jointly enabling smoother and brighter copper electrodeposits than either alone.

GEORGE W. JERNSTEDT.

JAMES D. PATRICK.

References Cited in the file of this patent UNITED STATES PA'I'ENTS Number Name Date 2,146,439 Oplinger 1- Feb. '7, 1939 2,287,654 Wernlund et a1. June 23, 1942 2,347,448 Wernlund .Apr. 25, 1944 2,402,185 Schweikher June 18, 1946 2,451,341 Jernstedt Oct. 12, 1948 2,524,912 Jernstedt Oct. 10, 1950 OTHER REFERENCES Ser. No. 351,241, Weiner (A. P. 0.), published May 18, 1943.

Meyer et al., Transactions of the Electrochemical Society, vol. 73 (1938) pp. 384, 400, 401. 

3. AN AQUEOUS CYANIDE ELECTROPLATING ELECTROLYTE COMPOSED OF FROM 2 TO 15 OUNCES PER GALLON OF COPPER, ALKALI METAL CYANIDE IN AN AMOUNT TO COMPLEX THE COPPER IN SOLUTION AND TO PROVIDE FROM 0.5 TO 5 OUNCES PER GALLON OF FREE ALKALI METAL CYANIDE, FROM 2 TO 8 OUNCES PER GALLON OF ALKALI METAL HYDROXIDE, FROM 1 TO 20 OUNCES PER GALLON OF ALKALI METAL CARBONATE, FROM 0.004 TO 0.57 OUNCE PER GALLON OF ZINC AND FROM 0.1 TO 10 OUNCES PER GALLON OF THIOCYANATE, THE ZINC AND THIOCYANATE JOINTLY ENABLING SMOOTHER AND BRIGHTER COPPER ELECTRODEPOSITS THAN EITHER ALONE. 